(a) Field of the Invention
The present invention relates to an optical film prepared from a polycycloolefin, and more particularly to an optical anisotropic-compensation film comprising a polycycloolefin, having negative birefringence along the thickness direction (nx≈ny>nz; nx=refractive index along the slow axis; ny=refractive index along the fast axis; ny=refractive index along the fast axis; nz=refractive index along the thickness direction), and a method for preparing the same.
(b) Description of the Related Art
Use of liquid crystal displays (LCDs) is on the rapid increase, since they consume less power and are, thereby capable of running for hours using a battery, they save space, and are more lightweight than cathode ray tube (CRT) displays. Recently, use of medium-to-large sized LCDs has been on the increase in computer monitors and TVs. Particularly, in medium-to-large sized LCDs, it is important to offer good image quality over a wide view angle and to improve contrast when the driving cell is turned ON/OFF.
For this reason, a variety of liquid crystal mode displays, such as dual domain TN, ASM (axially symmetric aligned microcell), VA (vertical alignment), SE (surrounding electrode), PVA (patterned VA), and IPS (in-plane switching), have been developed. Each of these modes has its own liquid crystal arrangement and unique optical anisotropy. Accordingly, a variety of compensation films are required to compensate for the linearly polarized light's change in the optical axis due to the optical anisotropy of liquid crystals in LCDs.
The compensation film plays an important role in solving light leakage of the vertical polarizing element at about 45° from the optical axis, as well as in optically compensating the optical anisotropy of liquid crystals in LCDs. Therefore, development of an optical film capable of accurately and effectively controlling the optical anisotropy is the most important factor for optical compensation of a variety of liquid crystal display modes.
The optical anisotropy is expressed in Rth, which is the phase difference along the fast axis (y-axis) and along the thickness direction (z-axis), and Re, which is the in-plane phase difference, as shown in the following Equation 1 and Equation 2:Rth=Δ(ny−nz)×d  Equation 1Re=Δ(nx−ny)×d  Equation 2
In Equations 1 and 2,
nx is the in-plane refractive index along the machine direction or along the slow axis (x-axis), ny is the in-plane refractive index along the transverse direction or along the fast axis (y-axis), nz is the refractive index along the thickness direction (z-axis), and d is the film thickness.
If any of Rth or Re is much larger than the other, the film can be used as a compensation film having uni-axial optical anisotropy, and if both of them are not close to 0, the film can be used as a compensation film having bi-axial optical anisotropy.
Compensation films having uni-axial optical anisotropy can be classified into the A-plate (nx≠ny≈nz) and the C-plate (nx≈ny≠nz). The in-plane phase difference can be controlled by such secondary film processing as precise film stretching, and thus optical isotropic materials can be uni-axial oriented. However, the controlling optical anisotropy along the thickness direction by secondary processing is relatively limited, and it is preferable to use a transparent polymer material having different molecular arrangements along the thickness direction and the planar direction. In particular, when considering compensation along the optical axis only, an ideal compensation film should have an optical axis which is a mirror image of that of the liquid crystal layer, and thus the negative C-plate having negative birefringence along the thickness direction can be required for VA mode and TN mode, which have higher refractive indices along the thickness direction than the planar direction.
Because the negative C-plate has a very small Re value, Rth can be obtained from the following Equation 3 by measuring Rθ, which is expressed by the product of optical path length and Δ(ny−nθ), the difference of refractive index along the fast axis and refractive index when the angle between the film plane and the incident ray of light is large:
                              R          th                =                                            R              θ                        ×            cos            ⁢                                                  ⁢                          θ              f                                                          sin              2                        ⁢                          θ              f                                                          Equation        ⁢                                  ⁢        3            
In Equation 3, θf is the internal angle.
For polymer materials that can be used as the negative C-plate, a discotic liquid crystal (e.g., U.S. Pat. No. 5,583,679), a polyimide having a planar phenyl group at the main chain (e.g., U.S. Pat. No. 5,344,916), and a cellulosic film (e.g., WO 2000/55657) are disclosed.
Of these materials, the discotic liquid crystal cannot be used alone and requires precise coating of up to several micrometers thickness on a transparent support. In addition to the cost of the coating process, the relative large birefringence of the discotic liquid crystal results in a relatively large phase difference as a result of a small difference in coating thickness, and pollutants such as dust remaining on the coating film surface or in the discotic liquid crystal solution may cause optical defects.
The polyimide is problematic because it experiences optical loss as it absorbs light in the visible region, and it peels easily due to weak adhesivity and high water absorptivity.
The cellulose ester-based film has problems in dimensional stability and adhesivity due to high water absorptivity, and is disadvantageous in durability compared with cycloolefin polymers due to the relatively high content of phase retarder compound having a low molecular weight. Also, resins comprising such an aromatic phase retarder compound have a relatively large wavelength dispersive characteristic due to the absorption in the visible region, which is seen from Sellmeyer's formula expressed by the following Equation 4:
                                          n            2                    ⁡                      (            λ            )                          =                  1          +                                                    A                1                            ⁢                              λ                2                                                                    λ                2                            -                              λ                1                2                                              +                                                    A                2                            ⁢                              λ                2                                                                    λ                2                            -                              λ                2                2                                              +          …                                    Equation        ⁢                                  ⁢        4            
In Equation 4,
n is the refractive index, λ1, λ2, . . . are absorption wavelengths, and A1, A2, . . . are fitting parameters.
Therefore, for a polymer material comprising an aromatic compound that is to be used as a compensation film, compensation of the wavelength dispersive characteristic should be considered because the phase difference varies a lot depending on the wavelength. That is, even if a compensation film comprising such material is optimized for optical compensation near 550 nm, where the highest optical efficiency is obtained, there arises a coloration problem because optical compensation is not satisfied for other wavelengths. This problem makes it difficult to control the display color.
On the contrary, since a cycloolefinic polymer does not absorb light in the visible region, it has a flat wavelength dispersive characteristic, and thus results in small phase differences with respect to wavelength. Cycloolefinic copolymers are well known in the literature. They have low dielectric constants, and low water absorptivity due to high hydrocarbon content.
For methods of polymerizing a cyclic monomer, there are ROMP (ring opening metathesis polymerization), HROMP (ring opening metathesis polymerization followed by hydrogenation), copolymerization with ethylene, and homogeneous polymerization, as seen in the following Scheme 1. Scheme 1 shows polymers having different structures and physical properties that are obtained from the same monomers depending on which polymerization method is applied.

The polymer obtained by ROMP has poor heat stability and oxidative stability due to unsaturation of the main chain, and is used as a thermoplastic resin or thermosetting resin. But the resin prepared from this method has a heat stability problem. Hydrogenation generally increases the glass transition temperature of the ROMP polymer by some 50° C., but the glass transition temperature is still low due to the ethylene group present between the cyclic monomers (Metcon 99).
In addition, cost increases due to more synthesis steps and weak mechanical properties are barriers to commercial utilization of such polymers. It has been reported that a polymer having a large molecular weight but a narrow molecular weight distribution can be obtained if a zirconium-based metallocene catalyst is used (Plastic News, Feb. 27, 1995, p.24). However, the activity decreases as the cyclic monomer concentration increases, and the copolymer has a low glass transition temperature (Tg<200° C.). Also, although the heat stability is improved, the mechanical strength is weak and chemical resistance against solvents or halogenated hydrocarbons is not good.
A cycloolefinic polymer obtained by addition polymerization using a homogeneous catalyst has a rigid and bulky ring structure in every monomer unit of the main chain. Thus, the polymer has very high Tg, and is amorphous. Therefore, the polymer neither experiences optical loss due to scattering nor absorbs light in the visible region by π-conjugation. Particularly, a cycloolefinic polymer having a relatively large molecular weight, which is obtained by addition polymerization using an organometallic compound as a catalyst, is electrically isotropic and has a low dielectric constant (J. Appl. Polym. Sci. Vol. 80, p 2328, 2001).
Thus, polymers prepared using norbornene monomers have high transmittivity, low birefringence, and high Tg, so they can be used for optical purposes such as in light guide panels and optical discs. Also, due to low dielectric constants, superior adhesivity, electrical isotropy, and high Tg, they can be used as insulation materials.
Introduction of substituents to a polymer comprising hydrocarbons is a useful method to control chemical and physical properties of the polymer. However, when introducing a substituent having a polar functional group, a free electron pair of the polar functional group tends to react with the active catalytic site and functions as a catalyst poison. Therefore, it is not always easy to introduce a polar functional group to a polymer, and there is a limit to the kind and amount of substituents that can be introduced. It is known that polymers prepared from substituted cyclic monomers have small molecular weights. In general, norbornene-based polymers are prepared by using post-transition organometallic catalysts. Most of such catalysts show low activity in polymerization of monomers containing polar groups, and generally, the prepared polymers have molecular weights of not more than 10,000 (Risse et al., Macromolecules, 1996, Vol. 29, 2755-2763; Risse et al., Makromol. Chem., 1992, Vol. 193, 2915-2927; Sen et al., Organometallics 2001, Vol 20, 2802-2812; Goodall et al., U.S. Pat. No. 5,705,503).